The present invention relates generally to the reduction of vortex-induced vibration (xe2x80x9cVIVxe2x80x9d) and more particularly to the reduction of VIV on pipes or other structural components immersed in a fluid.
Exploration for oil and natural gas reserves led drillers offshore many years ago and as offshore exploration continues, drillers find themselves in deeper and deeper waters. While those waters may bring the reserves they seek, the drillers are also faced with stronger currents threatening the structural integrity of their risers, pipelines, and other elongated components involved in oil and gas production.
Stresses on the pipes or other structural components immersed in fluid, such as a drilling riser, greatly increase as the velocity of the current increases and the stresses are magnified as the depth of the water at the well location increases. When operating drilling rigs in high current areas, the riser is exposed to currents that can cause at least two kinds of stresses. The first is caused by vibration resulting from vortices shed off a component when fluid flows by it. That vibration, occurring perpendicular to the current, is referred to as xe2x80x9cvortex-induced vibration,xe2x80x9d or xe2x80x9cVIV.xe2x80x9d When water flows past the riser, vortices are alternatively shed from each side of the riser. This produces a fluctuating force that the riser transfers to the current. If the frequency of this harmonic load is near the resonant frequency of the riser, large vibrations transverse to the current can occur. The second type of stress is caused by the drag forces that push the riser in the direction of the current due to the riser""s resistance to fluid flow. The drag forces are amplified by the vortex-induced vibrations of the riser. A riser pipe that is vibrating due to vortex shedding will disrupt the flow of water around it more than a stationary riser. This results in more energy transfer from the current to the riser, and hence more drag.
Shrouds, strakes and fairings have traditionally been added to drilling risers and other submerged pipes in order to minimize the current-induced stresses on these pipes. Strakes and shrouds can be effective regardless of the current orientation, but they tend to increase the drag acting on the riser. By contrast, fairings are generally more effective in reducing drag and VIV. Fairings generally comprise streamlined shaped bodies that weathervane or rotate about the riser maintaining positions substantially aligned with the water current. Fairings generally reduce vortex-induced forces and minimize drag on the riser by reducing or breaking up the low pressure areas that exist down-current of the riser.
One example of a fairing is found in U.S. Pat. No. 4,474,129 that discloses a fairing removably mountable on risers equipped with buoyancy modules that has a tail tapering aft and a fin positioned after the tail. This fairing completely surrounds the riser and is fastened together at the back portion of the fairing. Another example of a fairing is found in U.S. Pat. No. 4,398,487 which describes a streamlined symmetrical structure having a nose portion, a tail portion and two opposed side portions. This fairing is formed as two shell halves that completely surround the riser and are connected at the front end of the nose by quick release fasteners and the end of the tail portion by hinges. U.S. Pat. No. 5,410,979 describes a small fixed teardrop-shaped fairing that surrounds a riser and is fixed to the riser so as to not rotate. U.S. Pat. No. 6,048,136 describes a fairing that is installed on a drilling riser in combination with a synthetic foam buoyancy module. This fairing is formed as two shell halves that surround the riser and attach at the front and back portions of the fairing. A rotating fairing is described in U.S. Pat. No. 6,067,922 as including a copper element mounted in the annular region of the fairing to discourage marine growth. This fairing is formed as a single piece that completely surrounds the riser and is attached at the tail or flange portion with bolts. Another known fairing or strake 2, shown in FIG. 2, is constructed of fiberglass or polyurethane in two molded halves 4A and 4B that are strapped around the circumference of the riser. Asymmetrical fins 6 are molded along the sides of this strake. It should be noted that all of the above described fairings are constructed in predetermined lengths and a plurality of fairings are positioned along the length of any particular riser.
While fairings can be effective for reducing VIV, a number of problems still exist with the prior art fairings. As illustrated in the prior art, fairings have become more and more complex in design, they often require a large number of parts, and as such, they have become more costly to produce and maintain. Generally, fairings must be secured to the elongated component by bands, bolts or other fasteners that may fail. Further, the use of such fasteners adds to the cost and labor associated with the fairing""s use. Additionally, corrosion and marine growth frequently causes the rotational elements of a fairing to seize up so that it can no longer properly align with the current. Such a concern has often resulted in fairings being used only on risers or other components that remain on the risers only a short period of time, leaving those in the industry to rely upon less effective VIV reduction means for more permanently fixed components.
Fairings are typically applied to drilling risers in one of two ways. In one manner of installation, fairings are placed on the drilling riser after it is in place, suspended between the platform and the ocean floor, in which divers or submersible vehicles are used to fasten the multiple fairings around the drilling riser. A second method of installation is carried out as the riser is being assembled on a vessel. In this method the fairings are fastened to the pipe as lengths of pipes are fitted together to form the riser. This method of installation is performed on a specially designed vessel, called an S-Lay or J-Lay barge, that has a declining ramp, positioned along a side of the vessel and descending below the ocean""s surface, that is equipped with rollers. As the lengths of pipe are fitted together, fairings are attached to the connected pipe sections before the pipe is rolled down the ramp and into the ocean. One of the problems of installing fairings in this manner is that when the fins of the fairing rotate over the rollers on the ramp, the fins frequently become damaged by the rollers. In this method of installation, the completed drilling riser is pulled up to a vertical position when it reaches the appropriate length and is attached to the surface platform and the well head on the ocean floor.
It would be advantageous to provide a relatively lightweight, resilient fairing that can be easily snaked in place on a riser rather than having to be fastened around the entire circumference of a riser.
It would also be advantageous to have a fairing that rotates around the riser and takes advantage of natural boundary layer formation in the reduction of VIV on risers.
It would further be advantageous to provide a fairing that allows for pressure equalization and a reduction of marine growth.
The present invention is directed to a fairing system for the reduction of vortex-induced vibration and minimization of drag about a substantially cylindrical element immersed in a fluid medium. The fairing system includes a plurality of cylindrical shells rotatably mounted about a cylindrical element immersed in a fluid medium. Each cylindrical shell has opposing edges defining a longitudinal gap configured to allow the shells to snap around the cylindrical element. The longitudinal gap has a circumference of about 120xc2x0 relative to the circumference of each shell. Alternatively the longitudinal gap can have a circumference of about 60xc2x0 relative to the circumference of each shell. The shells also include a fin positioned along the each opposing edge of the longitudinal gap, in which each fin extends outwardly from each shell. The fins are positioned on each shell so as to reduce vortex-induced vibration and minimize drag on the cylindrical element.
The inventive fairing can be constructed from a non-metallic, low corrosive material such as fiberglass, polyurethane, vinyl ester resin, PVC or other materials having substantially similar flexibility and durability properties.